In a vehicle-to-vehicle communication, a technology has been used for changing a transmission interval of vehicle information from one vehicle to another vehicle so that each vehicle has enough chance to perform the vehicle-to-vehicle communication. For example, JP 2010-219919 A discloses an on-board communication device that extends a transmission interval of vehicle information in a stationary state of the vehicle so that the transmission interval in the stationary state is longer than a transmission interval in a travelling state of the vehicle. This is because, when the subject vehicle is in the stationary state, the subject vehicle is less likely to affect vehicles existing around the subject vehicle.
The transmission interval may be extended when a communication interruption occurs in a communication pathway caused by an obstruction existing in the communication pathway. An extension of the transmission interval may also be caused by a fading of a communication signal, or by a substantially long distance between two communication partners. Thus, in the vehicle-to-vehicle communication, when the transmission interval of vehicle information is extended at a first vehicle that transmits the vehicle information, a second vehicle that receives the vehicle information via the vehicle-to-vehicle communication from the first vehicle is difficult to distinguish an intentional extension of the transmission interval at the first vehicle from an extension of the transmission interval caused by an external factor. Thus, with the technology disclosed in JP 2010-219919 A, the second vehicle that receives the vehicle information from the first vehicle has a difficulty in determining an occurrence of the communication interruption in the vehicle-to-vehicle communication.
When a determination of the communication interruption occurrence becomes difficult at the second vehicle, the second vehicle may fail to provide a proper drive assist. Herein, the drive assist is performed based on the vehicle information transmitted from vehicles existing around the second vehicle. For example, the drive assist may include a determination and a notification of a vehicle that approaches to the second vehicle. When the determination of the communication interruption occurrence becomes difficult at the second vehicle, the second vehicle may fail to determine whether a vehicle exists around itself. When the second vehicle fails to specify a vehicle existing around itself, a proper drive assist cannot be provided to a driver or a user of the second vehicle.